An eye for an eye - vendettas originate within group

"It thus seems that vendettas may have a social function. They regulate how conflict is coped with, and can prevent unjust or excessive punishment. Vendettas can occur in the animal kingdom as well. Japanese macaques do not take revenge directly against an individual who has punished them, but instead against its relatives. They thereby diminish the danger of fresh revenge."

 http://www.sciencedaily.com/releases/2012/10/121024101650.htm?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+sciencedaily%2Fmind_brain+%28ScienceDaily%3A+Mind+%26+Brain+News%29